Portraits will APPEAR to change if you click another mission-giver on the missions tab, and then click back. This will not actually change the resulting Tech-Priests assigned portrait. It seems to keep the temporary portrait until a save is loaded. After which the original portrait that was generated will return. This is not how we force a portrait. It may be an hour (or a day) before you discover that the portrait reverted. Prevent yourself the frustration and use the method below

Portraits are generated for a Tech-Priest when the mission that gives them to you is generated.

Missions are generated when you complete (or fail) a mission.

1. Since we don't know when we will be offered a new Tech-Priest, save at the last possible moment in EVERY mission. Missions end after you kill the last enemy, scan the last terminal, or move the last Tech-Priest into the excape 'box', in the last objective room on the mission. Note: Sometimes, but not always, the game will create an auto-save OUTSIDE the combat, but still in the mission. If it happens to do this then you will want to use this save. This will make loading faster since you only have to go from Ship > Room Map instead of Ship > Combat Map > Combat Action/Animation > Room Map.
   
2. Check your missions. If you get a mission with a Tech-Priest and it is a duplicate portrait(or you just want a different one), reload the save and re-complete the mission.

Remember that the portrait that was generated with the mission is the one that will stick. If you load a save, whether on the ship or in the mission for the Tech-Priest, it will generate a 'fake' portrait. So when you get back to the ship after completing the mission and see you have been given some other random portrait, Just make a save and reload it. When you get back into the game the portrait should have reverted to the one that was first generated.